1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and more particularly to a method for forming a fine pattern consisting of an insulating film, a metallic film or the like.
2. Description of the Prior Art
In the manufacturing process of a semiconductor device, a mask consisting of a photoresist is in general use in order to form a pattern by etching a film to be etched such as an insulating film, a metallic film or the like deposited on a semiconductor substrate. Namely, after forming a photoresist film on a film to be etched, a mask consisting of a photoresist is formed by selectively irradiating the film with ultraviolet rays, developing it, and heating it at a temperature of about 130.degree. C.
In recent years unevenness on the surface of a semiconductor substrate is becoming more conspicuous due to complexity and refinement of the semiconductor device. In order to form a fine film pattern by etching with high accuracy, it is necessary to use a thin photoresist film which enables one to produce an accurate mask. However, when the unevenness of the substrate surface is large, the patterning accuracy of the photoresist film is deteriorated due to variation in the thickness of the photoresist film on the substrate surface caused by the unevenesss, impairing the patterning accuracy of the film to be etched. As a countermeasure to this problem, there has been examined a pattern formation method by means of a resist system with tri-level structure. In the resist system with tri-level structure a coating film such as spin on glass (SOG) is mainly used as the intermediate layer.
In the method that uses a resist system with tri-level structure, first, unevenness on a film to be etched is flattened by coating it with a photoresist film of about 2 .mu.m thickness, and then heated at 200.degree. C. for one hour to form a lower resist film. Next, an intermediate layer consisting of an SOG film is formed by coating a solution having silanol as a principal component to a thickness of about 0.2 .mu.m, then curing the coating by heating it at 150.degree. C. for one hour. Following that, after forming an upper resist film of thickness 0.3 .mu.m, the upper resist film is patterned by photolithography. Next the intermediate layer is etched by reactive ion etching (RIE) using CF.sub.4 gas with the upper resist layer as the mask. Further, the lower resist film is dry etched using O.sub.2 gas with the intermediate layer as the mask. By the process in the above, a mask consisting of a lower resist film with improved dimensional accuracy can be obtained.
However, the aforementioned method of forming a mask consisting of a lower resist film using the resist system of tri-level structure, has the following problems.
Namely, the coating film used as the intermediate film develops cracks or delaminations at the time of its heat treatment due to volume shrink by heating or mismatch in the coefficient of thermal expansion with the lower resist film. Moreover, for heat treatment of the coating film at a temperature below 200.degree. C., perfect condensation reaction does not take place so that sufficient strength can no longer be obtained. Because of these, the coating film is delaminated and becomes the cause of generation of particles.
Furthermore, when the film to be etched is a film of a noble metal such as gold, there is a problem in that heating of the lower resist film at a temperature above 150.degree. C. causes the delamination of the film. This results in the deterioration of the production yield and the reliability of the semiconductor device.